The present invention pertains to semiconductor integrated circuits. This invention particularly pertains to flip chip integrated circuits, also known as flip chips. This invention more particularly pertains to light sensitive flip chips and to a flip chip structure that excludes ambient light. The manufacturing process for making integrated circuit chips is performed, not on individual integrated circuit chips, but rather on semiconductor wafers. At the present state of the art many thousands of integrated circuits may be formed on a silicon wafer whose diameter is eight inches. Each circuit is a structure that includes layers of insulator, conductor and semiconductor, each layer being patterned according to a circuit plan. Bonding pads are formed on the surface of the wafer so that each integrated circuit has bonding pads for input signals, output signals, supply voltage, and ground. The wafer is finally cut into a number of integrated circuit chips. Subsequently, each integrated circuit chip is individually processed, either being installed in an integrated circuit package, or being attached to some form of substrate.
In the manufacture of flip chips, some additional process steps are performed on the wafer, prior to the operation of cutting the wafer into individual flip chips. These process steps provide an elevated bump on each bonding pad. Typically, these raised bumps are solder bumps.
A flip chip is attached to a substrate, such as a printed circuit board, by bringing the solder bumps into contact with metal traces on the substrate and then, in one operation, reflowing the solder bumps so as to permanently attach solder bumps to traces. FIG. 1 shows a cross section of a flip chip 10 attached to a substrate 11. Each solder bump 12 is attached to a metal trace (not shown) on the substrate 11. After the flip chip 10 is attached to the 10 substrate 11, the space between the flip chip and the substrate is commonly filled with a material known as underfill 13. The underfill material is selected for its mechanical properties so that it redistributes the mechanical and thermnomechanical stresses arising between the flip chip 10 and the substrate 11. Thus the underfill 13 protects the flip chip 10 from mechanical damage.
In copending application Ser. No. 09/031,167 an invention is disclosed in which protection from mechanical damage is provided in a different way from that just described. A resilient protective layer is applied to the semiconductor wafer before the solder bumps are formed. After the flip chip has been attached to a substrate, the resilient protective layer protects the solder bumps and adjacent regions of the flip chip from mechanical damage arising from mechanical and thermnomechanical stresses arising between the flip chip and the substrate. The resilient layer provides protection against mechanical damage while avoiding the use of underfill. Thus the expense of providing underfill is avoided.
Now, without underfill, the circuit side of the flip chip is exposed to whatever ambient light enters the space between flip chip and substrate. Ambient light includes natural daylight and lamplight. Some flip chips can operate satisfactorily in ambient light. However, some other flip chips are so sensitive to light that their electrical characteristics are altered to an unacceptable degree. These light sensitive flip chips must be isolated from ambient light. Thus a need exists for protection for light sensitive flip chips from ambient light.